As a technique of obtaining an extremely fine grained crystal grain of steel in order to improve the mechanical property of steel material, in manufacturing a hot rolled-steel sheet, a method for rolling a steel sheet at a high rolling reduction at the time of finish rolling and then rapidly cooling the steel sheet just after the finish rolling is suggested. In connection with this, for example as the cooling apparatus disclosed in Patent Document 1, a technical development to establish both a high cooling speed and a uniform cooling (cooling uniformity) has been proceeded (hereinafter, to rapidly cool a steel sheet just after finish rolling is sometimes referred to as “rapidly cool(ing) immediately after finish rolling”, and the cooling apparatus for the rapid cooling is sometimes referred to as “apparatus for rapid cooling immediately after (rolling)”).
Here, in an actual manufacturing line of a hot-rolled steel sheet, in many cases, a steel sheet in which the crystal grain is extremely fine grained and a general hot-rolled steel sheet (ordinary material) are manufactured in a same line, thus the above-mentioned apparatus for rapid cooling immediately after rolling is not always used. Therefore, an on-off mechanism which switches on and off of spraying is provided to the apparatus for rapid cooling immediately after rolling.
For example in a case where the ordinary material is continuously manufactured for a long time because of conditions of manufacturing schedule of steel sheet, sometimes the apparatus for rapid cooling immediately after rolling is not used for a long time. In this case, because of the radiation heat from a high-temperature steel sheet (800° C. to 900° C.) which comes through a guiding sheet, a deformation originated from heat strain is occurred to the spray nozzle; thus it is concerned that the steel sheet is not uniformly cooled over time. In order to prevent such a heat strain, in a case where the apparatus for rapid cooling immediately after rolling is not used, or the hot-rolled steel sheet is continuously manufactured without using a part of the apparatus for rapid cooling immediately after rolling, it can be considered to cool inside the spray nozzle by spraying water from the spray nozzle for approximately 15 to 20 seconds from the end of the rolling of the preceding steel sheet to the start of the rolling of the following steel sheet.
However, in this case as well, it is not possible to spray pressurized water in rolling, and the spray nozzle is subjected to a large amount of radiation heat from the high-temperature (800° C. to 900° C.) steel sheet, whereby there is a possibility that the deformation of the spray nozzle cannot be inhibited because of repeated cooling and heating.
Also, in a case where the use and disuse of the spray nozzle are switched, an on-off valve can be provided to a water-supply pipe which supplies pressurized water to the spray nozzle. However, if the start/stop of spraying of cooling water is controlled by such an on-off valve, especially at the header on an upper surface side of the steel sheet, the cooling water accumulated to a pipe positioned between the on-off valve and the spray nozzle flows out from the spray nozzle due to gravity, when the spraying of cooling water is stopped. Then, when the on-off valve is opened and the spraying of cooling water is started next time, the spraying of cooling water is not started until the cooling water is filled in a portion from which the water flown out. This causes a problem that the time lag from the order of spraying cooling water to the actual start of the spraying becomes large. This kind of time lag can be a delay or variability of cooling which causes variability in the property of the steel sheet.
From such a viewpoint, it is preferable that the on-off valve is provided to each spray nozzle. According to this, it is possible to dissolve the time lag described above. In this regard, for example an on-off valve described in Patent Document 2 or 3 can be used.
However, radiation heat from the high-temperature (800° C. to 900° C.) steel sheet is irradiated to the spray nozzle via the guiding sheet as described above. Therefore, in a case where the on-off valve is used for the spray nozzle, there is a need to protect each member configuring the on-off valve from the radiation heat. Especially, a member comparatively weak to heat, such as a sealing material, is also provided to the on-off valve; therefore the effect from heat is a problem which occurs not only with time passes, but also in a short period.